A constant velocity universal joint for automobiles, as an important member for connecting and transmitting a driving shaft between an output shaft of a gearbox and a wheel hub, can be classified into a fixed constant velocity universal joint and a telescopic sliding constant velocity universal joint according to operating properties of the universal joint. A tripod universal joint is one of the most common forms in the telescopic sliding constant velocity universal joint.
The tripod universal joint is composed of a three-column groove shell, a retainer ring, bearing rings, roller pins and a tripod. A main function of the tripod universal joint is to connect two shafts between which an included angle is formed or of which mutual positions are changed, so that the two shafts transmit power at the same angular velocity and have a certain displacement capacity in an axial direction.
The tripod is composed of three pin shafts with an included angle of 120 degrees. The bearing rings are mounted on the pin shafts through the roller pins. Three chutes (fairways) along an axial direction of the groove shell are formed at an inner cavity of the three-column groove shell, and the three-column groove shell is installed in coordination with the three bearing rings and can be axially moved along the chutes. When a certain pivot angle exists between the three-column groove shell and the tripod, after a certain torque is applied to the three-column groove shell, the torque is transferred to the tripod by the bearing rings so as to drive the tripod to rotate. The tripod slides in and out along an axis of the three-column groove shell in a steering travel process to compensate an axial length change of the universal joint.
Based on friction and extrusion caused by the bearing rings and the chutes in a relative motion process, contact surfaces of the chutes and the bearing rings of the tripod universal joint are easy to be worn and deformed, so surface hardness and wear resistance of the contact surfaces are improved in a manner of performing heat treatment on a metal surface. The chutes in the inner cavity of the three-column groove shell are often subjected to heat treatment in an intermediate frequency quenching manner in a current industry.
Heat treatment to the inner cavity of the tripod universal joint above is performed in the intermediate frequency quenching manner for improving the mechanical property of a friction surface. Technical needs after heat treatment of the universal joint can be met in improvement of material hardness and wear resistance in an existing technical solution. However, an intermediate frequency quenching principle determines that a material after heat treatment may cause a deformation phenomenon. The intermediate frequency quenching causes deformation for two main reasons: (1) an internal structure of the material is transformed from austenite to martensite after the intermediate frequency quenching, and a specific volume of the martensite is larger than that of the austenite, so an operation of performing intermediate frequency quenching and cooling on a workpiece is a volume expansion process, called as specific volume deformation; and (2) different heat expansion and cold contraction degrees are caused due to non-uniform heating and cooling in the heat treatment process, an internal stress is further produced, and deformation produced by the internal stress is called internal stress plastic deformation.
In an ideal state, an inner outline of a section of the chutes of the tripod universal joint is of an inverted olive shape, namely, sizes of an opening and a bottom are larger, while a size of a middle is smaller. This structure is favorable for smoothly mounting the bearing rings into the chutes and can ensure that the tripod has a certain motion amount along the chutes.
Inclination of the chutes should be less than 0.02, that is, the chutes are in a shape of a horn, after the tripod universal joint is finished. According to past experience, the deformation produced after the intermediate frequency quenching may cause that the inner outline of the section of the chutes presents an olive shape with a slightly bulged middle and two narrow ends.
In the industry, an opening area of each chute is called section a, an intermediate working area is called section b, and a bottom area is called section c. In order to guarantee convenient installation and reliable transmission, a groove width of the section a and a groove width of the section c are generally required to be less than a groove width of the section b. An assembling clearance size of the section b of each chute and each bearing ring is controlled to be 0.08 mm-0.18 mm. A problem brought by deformation of the inner cavity is that the three bearing rings cannot be installed into the chutes according to set sizes, so an outer diameter of each bearing ring needs to be polished to be small enough to pass through the section a area of the opening before the bearing ring is assembled. However, a problem brought by this manner is that an assembling clearance between the polished bearing ring and the section b working area is too large and exceeds a reasonable range. In a loading travel process, abnormal sound and vibration during transmission are caused due to too large clearances between the bearing rings and the chutes of the universal joint, and even premature failure of the universal joint may be caused after a long time.
At present, the universal joint often presents an olive structure with the slightly bulged middle and two narrow ends after the intermediate frequency quenching, while the olive structure is far different from the expected inverted olive structure. Because deformation after intermediate frequency quenching is related to intensity of a generated alternating magnetic field, the deformation herein after the intermediate frequency quenching may be correspondingly weakened as long as intensity of a certain magnetic field is weakened theoretically. Because a quenching inductor generates an alternating magnetic field by powering an induction coil by alternating current, the generated alternating magnetic field forms high-density induction current (eddy current) on the surface of the workpiece through an “induction heating principle”, and the formed induction current is changed into heat energy for heating the workpiece. For a tripod universal joint fairway, heat treatment is performed only on a contact surface between the fairway and each bearing ring, instead of the whole inner cavity, so silicon steel sheets need to be arranged at corresponding positions of the induction coil. The silicon steel sheets are “[”-shaped metal sheets, and each quenching surface is formed by superposing a group of silicon steel sheets. By virtue of magnetic conductive properties of the silicon steel sheets, when the current passes through the silicon steel sheets, a self-induced electromotive force is high due to high magnetic flux density of a core, and the current is driven to an opening side with small inductive reactance, that is, one side needing to be quenched, so that only the quenching surfaces are quenched, instead of all surfaces of the inner cavity.
An existing intermediate frequency quenching inductor includes a base and a heating inductor mounted on the base. An opening positioning device for positioning an opening of the tripod universal joint is mounted on the base of the heating inductor and is used for determining a position of the tripod universal joint during heat treatment. After used for a long time, the opening positioning device is easily rubbed by the opening of the tripod universal joint and worn, and overall positioning accuracy of the tripod universal joint is reduced, so the bottom of the tripod universal joint fairway is contacted with inductor accessories and worn, causing that the tripod universal joint is directly contacted with an effective coil to generate ignition and a copper tube of the effective coil is broken, thereby shortening service life of the quenching inductor.